There is, as yet, no means of converting a non-JAA licence to a JAA licence, nor is there likely to be for at least 4 years. In order to instruct in the UK your friend will need to obtain a JAA professional licence and an instructor rating. The path to these is long, complex and expensive.
Given that he currently has a CPL, and that he will ultimately wish to obtain a JAA ATP(A), he will have to achieve the following:
Complete an approved course of ground training (duration at the discretion of the Head of Training of an approved training organisation) and pass all 14 JAA ATP(A) theoretical knowledge examinations. (If he wishes to obtain only a CPL he must pass the appropriate CPL theoretical examinations)
Complete an approved course of flight training (duration at the discretion of...etc) and pass the JAA CPL Skill Test in a complex single or twin engined aeroplane.
Complete an approved course of flight training (duration at the discretion of.... etc) and pass the JAA Instrument Rating Skill Test.
Complete an approved course of 30 hours flight training and 125 hours ground training and pass the Flight Instructor (Restricted) Skill Test with a Flight Instructor Examiner.
He will get absolutely no credit for the instructional experience he has gained in the US.
Were he to first obtain a FAA ATPL, there would be no requirement to take the CPL or IR Skill Test, nor to complete approved courses of training. He would still have to pass all 14 of the JAA theoretical knowledge examinations and would have to pass the JAA ATP(A) Skill Test. This is taken on a multi-pilot aeroplane and, if your friend does not already have a suitable multi-crew type on his FAA licence, he would have to complete an approved type rating course before taking the Skill Test. Subsequently he would have to complete the FI(R) course as above.
Easy, innit?